1. Field of the Invention
The invention in general relates to electrosurgery, and in particular to an electrosurgical unit having a regulated power output.
2. Description of the Prior Art
Electrosurgical generators which produce high frequency electric currents used for cutting of tissue and coagulation of small blood vessels have been well-known for several generations. Since the amount of cutting or coagulation performed is related to the voltage and power output of the generator, it is also known that it is desirable that the output signal of the generator be regulated. By a regulated output signal it is meant that the signal is controlled so that the fluctuations in the line input voltage do not substantially affect the power output. By substantially, it is meant that the output voltage is sufficiently regulated so that differences in cutting or coagulation are not noticable by the surgeon for normal fluctuation levels within conventional AC voltage lines.
The most common manner of obtaining a regulated output is through the use of regulated power supplies. U.S. Pat. No. 3,963,030 issued David W. Newton, discloses a feedback circuit for regulating the output of an electrosurgical generator. This feedback circuit includes a current sensor which senses the current and the output circuitry and a comparator which compares the output voltage to a reference voltage and which then adjusts the amplifier to account for variations in the amplified signal. Since fluctuations in the amplified signal are proportional to the fluctuations of the supply voltage, this method provides the desired regulation. See also U.S. Pat. No. 4,092,986 issued to Max Schneiderman.
The prior art means for regulating the output of an electrosurgical unit have several disadvantages. A regulated power supply is relatively expensive, bulky, consumes significant electrical energy and produces significant heat. These disadvantages are especially accented when a high output supply, such as the supply used for the power output stages of an electrosurgical generator, are regulated. Since under actual use the output of an electrosurgical unit varies with changing conditions at or between its electrodes, any easily measured output parameter will also vary. For example, the voltage will change with the change in resistance between the electrodes. Thus such output parameters are not generally indicative of the variations in line voltage. As a result the feedback type of circuitry is not as reliable and effective as a feedforward compensated power supply. Moreover, oscillations can occur in output parameters that are controlled by feedback loops.